Wireless communication system using pilot subcarrier allocation

ABSTRACT

A wireless communication system using pilot subcarrier allocation and a method of allocating the pilot subcarriers for use in downlink and uplink communication in a multiple-input multiple-output (MIMO) antenna system using orthogonal frequency division multiplexing (OFDM) modulation are disclosed. The method includes providing a frame structure comprising OFDM symbols in time domain and subcarriers in frequency domain and alternately allocating first pilot subcarriers for a first antenna and second pilot subcarriers for a second antenna in the time and frequency domains, wherein each of the alternating first pilot subcarriers and the second pilot subcarriers is separated by multiple of 9 subcarriers in the frequency domain and further allocated in two contiguous OFDM symbols.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. §119(a), this application claims the benefit ofearlier filing dates and right of priority to Korean Application Nos.10-2007-0091730 and 10-2008-0078432, filed on Sep. 10, 2007 and Aug. 11,2008, respectively, and U.S. Provisional Application Nos. 61/021,894 and61/045,624, filed on Jan. 17, 2008 and Apr. 17, 2008, respectively, thecontents of which are hereby incorporated by reference herein in theirentirety.

FIELD OF THE INVENTION

The present invention relates to a wireless communication system. Inparticular, the present invention relates to a method of allocatingpilot subcarriers in the wireless communication system including amultiple-input multiple-output (MIMO) antenna system.

DESCRIPTION OF THE RELATED ART

The Institute of Electrical and Electronics Engineers (IEEE) 802.16standard provides a technology of supporting a broadband wireless accessand a protocol. The standardization has been progressed since 1999 andIEEE 802.16-2001 was approved in 2001. This has been established basedon a single carrier physical layer called “WirelessMAN-SC”. In the IEEE802.16a which was approved in 2003, “WirelessMAN-OFDM” and“WirelessMAN-OFDMA” were added to a physical layer in addition to“WirelessMAN-SC”. After the IEEE 802.16a standard was completed, arevised IEEE 802.16-2004 was approved in 2004. In order to correct bugsand errors of the IEEE 802.16-2004, IEEE 802.16-2004/Cor1 was completedin the form of “corrigendum” in 2005.

A MIMO antenna technology improves data transmission/receptionefficiency using multiple transmission antennas and multiple receptionantennas. The MIMO technology was introduced in the IEEE 802.16astandard and has been continuously updated.

The MIMO technology is divided into a spatial multiplexing method and aspatial diversity method. In the spatial multiplexing method, sincedifferent data is simultaneously transmitted, data can be transmitted ata high speed without increasing the bandwidth of a system. In thespatial diversity method, since the same data is transmitted viamultiple transmission antennas so as to obtain diversity gain,reliability of the data is increased.

A receiver needs to estimate a channel in order to restore datatransmitted from a transmitter. Channel estimation indicates a processof compensating for distortion of a signal, which occurs by a rapidenvironment variation due to fading and restoring the transmissionsignal. In general, for channel estimation, the transmitter and thereceiver need to know pilots.

In the MIMO system, a signal experiences a channel corresponding to eachantenna. Accordingly, it is necessary to arrange the pilots inconsideration of multiple antennas. While the number of pilots isincreased as the number of antennas is increased, it is impossible toincrease the number of antennas so as to increase a data transfer rate.

In the prior art, different pilot allocation structures have beendesigned and used according to permutation (dispersion/AMC) methods.This is because the permutation methods are separated from each other intime domain in the IEEE 802.16e system and thus structures differentlyoptimized according to permutation methods are designed. If thepermutation methods coexist in certain time instance, a unified basicdata allocation structure is necessary.

In the prior art, since severe pilot overhead occurs, a transfer rate isdecreased. In addition, since the same pilot structure is applied toadjacent cells or sectors, collision between the cells or the sectorsmay occur. Accordingly, there is a need for a method of efficientlyallocating pilot subcarriers in the MIMO system.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of efficientlyallocating pilot subcarriers in wireless communication system includinga MIMO system. The object of the present invention can be achieved byproviding a method of allocating pilot subcarriers for a plurality ofantennas in the MIMO antenna system over a plurality of orthogonalfrequency division multiplexing (OFDM) symbols and a plurality ofsubcarriers. The method includes allocating each of the pilotsubcarriers for the plurality of antennas by the same number in everyOFDM symbols. The pilot subcarriers form pairs of two pilot subcarriersand are allocated to be alternately arranged on two contiguous OFDMsymbols in a time domain.

According to an embodiment of the present invention, a method ofallocating pilot subcarriers for use in downlink and uplinkcommunication in the MIMO antenna system using OFDM modulation isprovided. The method includes providing a frame structure comprisingOFDM symbols in time domain and subcarriers in frequency domain andalternately allocating first pilot subcarriers for a first antenna andsecond pilot subcarriers for a second antenna in the time and frequencydomains, wherein each of the alternating first pilot subcarriers and thesecond pilot subcarriers is separated by multiple of 9 subcarriers inthe frequency domain and further allocated in two contiguous OFDMsymbols.

In an aspect of the present invention, each of the first pilotsubcarriers separated by two OFDM symbols is shifted by multiple of 3subcarriers, and each of the second pilot subcarriers separated by twoOFDM symbols is shifted by multiple of 3 subcarriers to differentiatefrequency selectivity. Each OFDM symbol includes first and second pilotsubcarriers. Preferably, number of each of the first and second pilotsubcarriers in each OFDM symbol is the same.

In another aspect of the present invention, the method further includesalternately allocating third pilot subcarriers for a third antenna andfourth pilot subcarriers for a fourth antenna in the time and frequencydomains, wherein each of the alternating third pilot subcarriers and thefourth pilot subcarriers are separated by multiple of 9 subcarriers inthe frequency domain and further allocated in two contiguous OFDMsymbols. Preferably, the first pilot subcarriers are adjacent to thethird pilot subcarriers in the frequency domain and the second pilotsubcarriers are adjacent to the fourth pilot subcarriers in thefrequency domain. Preferably, each of the third pilot subcarriersseparated by two OFDM symbols is shifted by multiple of 3 subcarriers,and each of the fourth pilot subcarriers separated by two OFDM symbolsis shifted by multiple of 3 subcarriers to differentiate frequencyselectivity.

The frame structure may be used in one of the uplink and downlinkcommunications. The frame structure may be used in a type of a fullusage of subchannels (FUSC) permutation mode. The frame structure may beused in a type of an adaptive modulation and coding (AMC) permutationmode.

In still another aspect of the present invention, a starting location ofthe first pilot subcarriers in a first OFDM symbol is offset by 1subcarrier. Each OFDM symbol may include first, second, third and fourthpilot subcarriers and preferably, number of each of the first, second,third and fourth pilot subcarriers in each OFDM symbol is the same.

According to an embodiment of the present invention, a wirelesscommunication system using orthogonal frequency division multiplexing(OFDM) modulation for downlink and uplink communication includes amultiple-input multiple-output (MIMO) antenna, an OFDM modulatoroperably connected to the MIMO antenna, and a processor operablyconnected to the OFDM modulator. The processor is configured to providea frame structure comprising the OFDM symbols in time domain andsubcarriers in frequency domain, and alternately allocate first pilotsubcarriers for a first antenna and second pilot subcarriers for asecond antenna in the time and frequency domains. Preferably, each ofthe alternating first pilot subcarriers and the second pilot subcarriersis separated by multiple of 9 subcarriers in the frequency domain andfurther allocated in two contiguous OFDM symbols.

In an aspect of the present invention, the processor includes asubcarrier allocator allocating symbols and pilots to the subcarriersfor use in at least one of the downlink and uplink communication. Theprocessor may further include a channel encoder encoding an input streamand building a coded word, a mapper mapping the coded word to a symbolrepresenting a position on signal constellation, and a multiple-inputmultiple-output (MIMO) processor processing the symbol.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

FIG. 1 is a block diagram of a transmitter having multiple antennas.

FIG. 2 is a block diagram of a receiver having multiple antennas.

FIG. 3 shows a frame structure.

FIG. 4 shows a pilot arrangement of two transmission antennas in partialusage of subchannels (PUSC).

FIG. 5 shows a pilot arrangement of two transmission antennas in fullusage of subchannels (FUSC).

FIG. 6 shows a pilot arrangement of four transmission antennas in thePUSC.

FIG. 7 shows a pilot arrangement of four transmission antennas in FUSC.

FIG. 8 is a graph showing cases of Table 2 by a ratio of pilot overheadand a ratio of guard subcarriers.

FIG. 9 shows the pilot arrangement of the two transmission antennas.

FIG. 10 shows the pilot arrangement of the four transmission antennas.

FIG. 11 illustrates the pilot arrangement of three or four transmissionantennas by equation.

FIG. 12 shows the pilot arrangement of the two transmission antennas.

FIG. 13 shows the pilot arrangement of the two transmission antennas.

FIG. 14 shows the pilot arrangement of the four transmission antennas.

FIG. 15 shows the pilot arrangement of the four transmission antennas.

FIGS. 16˜18 show some pilot subcarrier allocation structures in a systemusing eight antennas according to an embodiment of the presentinvention.

FIGS. 19 and 20 show embodiments of the present invention in which apilot subcarrier allocation offset varies according to cells in a 4-Txsystem and an 8-Tx system.

FIG. 21 shows another embodiment of the 8-Tx system shown in FIG. 20.

FIG. 22 shows a pilot subcarrier allocation pattern according to anembodiment of the present invention.

FIGS. 23˜25 show pilot subcarrier allocation patterns in a 2-Tx systemaccording to an embodiment of the present invention.

FIGS. 26 and 27 show pilot subcarrier allocation patterns in a 4-Txsystem according to an embodiment of the present invention.

FIG. 28 shows a pilot subcarrier allocation pattern in a 4-Tx systemaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description, reference is made to theaccompanying drawing figures which form a part hereof, and which show byway of illustration specific embodiments of the invention. It is to beunderstood by those of ordinary skill in this technological field thatother embodiments may be utilized, and structural, electrical, as wellas procedural changes may be made without departing from the scope ofthe present invention. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or similarparts.

The following technology may be used in various wireless communicationsystems. A wireless communication system is widely provided in order toprovide various communication services such as voice and packet data.This technology may be used in downlink or uplink. In general, thedownlink indicates communication from a base station (BS) to a userequipment (UE) and the uplink indicates communication from the UE to theBS. The BS generally indicates a fixed station communicating with the UEand may also be called a node-B, a base transceiver system (BTS) or anaccess point. The UE may be fixed or moved and may also be called amobile station (MS), a user terminal (UT), a subscriber station (SS) ora wireless device.

Hereinafter, an efficient pilot structure for a new system will bedescribed. The new system will be described concentrating on an IEEE802.16m system, but the same principle may apply to other systems.

A communication system may be a multiple-input multiple-output (MIMO)system or a multiple-input single-output (MISO) system. The MIMO systemuses a plurality of transmission antennas and a plurality of receptionantennas. The MISO system uses a plurality of transmission antennas andone reception antenna.

FIG. 1 is a block diagram of a transmitter having multiple antennas.Referring to FIG. 1, the transmitter 100 includes a channel encoder 120,a mapper 130, an MIMO processor 140, a subcarrier allocator 150 and anorthogonal frequency division multiplexing (OFDM) modulator 160. Thechannel encoder 120, mapper 130, MIMO processor 140, and subcarrierallocator 150 may be embodied as separate components or combined in asingle processor of the transmitter 100.

The channel encoder 120 encodes an input stream according to apredefined coding method and builds a coded word. The mapper 130 mapsthe coded word to a symbol representing a position on signalconstellation. A modulation scheme of the mapper 130 is not restrictedand may include an m-phase shift keying (m-PSK) scheme or anm-quadrature amplitude modulation (m-QAM) scheme.

The MIMO processor 140 processes the input symbol by a MIMO method usinga plurality of transmission antennas 190-1, . . . , and 190-Nt. Forexample, the MIMO processor 140 can perform preceding based on codebook.

The subcarrier allocator 150 allocates the input symbol and pilots tosubcarriers. The pilots are arranged according to the transmissionantennas 190-1, . . . , and 190-Nt. The pilots are known by both thetransmitter 100 and a receiver (200 of FIG. 2), which are used for thechannel estimation or the data demodulation, and are also calledreference signals.

The OFDM modulator 160 modulates the input symbol and outputs OFDMsymbols. The OFDM modulator 160 may perform an inverse fast Fouriertransform (IFFT) with respect to the input symbol and further insert acyclic prefix (CP) after performing the IFFT. The OFDM symbols aretransmitted via the transmission antennas 190-1, . . . , and 190-Nt.

FIG. 2 is a block diagram of a receiver having multiple antennas.Referring to FIG. 2, the receiver 200 includes an OFDM demodulator 210,a channel estimator 220, an MIMO post-processor 230, a demapper 240 anda channel decoder 250. The channel estimator 220, MIMO post-processor230, demapper 240 and channel decoder 250 may be embodied as separatecomponents or combined in a single processor of the receiver 200.

Signals received via reception antennas 290-1, . . . , and 290-Nr areFast Fourier transformed (FFT) by the OFDM demodulator 210. The channelestimator 220 estimates channels using pilots. The MIMO post-processor230 performs a post-process corresponding to the MIMO processor 140. Thedemapper 240 demaps the input symbol to the coded word. The channeldecoder 250 decodes the coded word and restores original data.

FIG. 3 is an example of a frame structure. A frame is a data sequenceduring a fixed time period used by a physical specification, whichrefers to section 8.4.4.2 of IEEE standard 802.16-2004 “Part 16: AirInterface for Fixed Broadband Wireless Access Systems” (hereinafter,referred to as reference Document 1).

Referring to FIG. 3, the frame includes a downlink (DL) frame and anuplink (UL) frame. Time division duplex (TDD) is a scheme in which theuplink and downlink transmission are separated in time domain but sharethe same frequency. Normally, the DL frame precedes the UL frame. The DLframe starts in the order of Preamble, Frame Control Header (FCH),Downlink (DL)-MAP, Uplink (UL)-MAP and burst regions (DL burst #1˜5 andUL burst #1˜5). A guard time for separating the DL frame and the ULframe from each other is inserted at both an intermediate portion of theframe (between the DL frame and the UL frame) and a last portion of theframe (following the UL frame). A transmit/receive transition gap (TTG)is a gap defined between a downlink burst and a subsequent uplink burst.A receive/transmit transition gap (RTG) is a gap defined between anuplink burst and a subsequent downlink burst.

The preamble is used for initial synchronization between the BS and theUE, cell search, frequency offset estimation and channel estimation. TheFCH includes information regarding the length of the DL-MAP message andthe coding scheme of the DL-MAP. The DL-MAP is a region where the DL-MAPmessage is transmitted. The DL-MAP message defines the access of adownlink channel. The DL-MAP message includes a configuration changecount of a Downlink Channel Descriptor (DCD) and a BS identifier (ID).The DCD describes a downlink burst profile applied to a current frame.The downlink burst profile refers to the property of a downlink physicalchannel, and the DCD is periodically transmitted by the BS through theDCD message.

The UL-MAP is a region where the UL-MAP message is transmitted. TheUL-MAP message defines the access of an uplink channel. The UL-MAPmessage includes a configuration change count of an Uplink ChannelDescriptor (UCD), and an effective start time of the uplink allocationdefined by the UL-MAP. The UCD describes an uplink burst profile. Theuplink burst profile refers to the property of an uplink physicalchannel, and the UCD is periodically transmitted by the BS through theUCD message.

Hereinafter, a slot is a minimum data allocation unit and is defined bya time and a subchannel. The number of subchannels depends on a FFT sizeand time-frequency mapping. The subchannel includes a plurality ofsubcarriers and the number of subcarriers per subchannel variesaccording to a permutation method. The permutation indicates mapping ofa logical subchannel to a physical subcarrier. The subchannel includes48 subcarriers in full usage of subchannels (FUSC) and the subchannelincludes 24 or 16 subcarriers in partial usage of subchannels (PUSC). Asegment indicates at least one subchannel set.

In order to map data to physical subcarriers in a physical layer, twosteps are generally performed. In a first step, data is mapped to atleast one data slot on at least one logical subchannel. In a secondstep, the logical subchannel is mapped to the physical subchannel. Thisis called permutation. Reference document 1 discloses a permutationmethod such as FUSC, PUSC, Optimal-FUSC (O-FUSC), Optional-PUSC (O-PUSC)and Adaptive modulation and coding (AMC). A set of OFDM symbols usingthe same permutation method is called a permutation zone and one frameincludes at least one permutation zone.

The FUSC and the O-FUSC are used only for the downlink transmission. TheFUSC is composed of one segment including all subchannel groups. Thesubchannels are mapped to the physical subcarriers distributed via allphysical channels. The mapping is changed according to OFDM symbols. Theslot is composed of one subchannel on one OFDM symbol. The methods ofallocating pilots in the O-FUSC and FUSC are different from each other.

The PUSC is used for both the downlink transmission and the uplinktransmission. In the downlink, each physical channel is divided intoclusters including 14 contiguous subcarriers on two OFDM symbols. Thephysical channel is mapped in the unit of six groups. In each group, thepilots are allocated to the clusters at fixed positions. In the uplink,the subcarriers are divided into tiles composed of four contiguousphysical subcarriers on three OFDM symbols. The subchannel includes sixtiles. The pilots are allocated to corners of the tiles. The O-PUSC isused only for the uplink transmission and the tile is composed of threecontiguous physical subcarriers on three OFDM symbols. The pilots areallocated to the centers of the tiles.

FIGS. 4 and 5 show a pilot arrangement of two transmission antennas inthe PUSC and FUSC, respectively. FIGS. 6 and 7 show a pilot arrangementof four transmission antennas in the PUSC and FUSC, respectively. Theyrefer to section 8.4.8.1.2.1.1, section 8.4.8.1.2.1.2, section8.4.8.2.1, and section 8.4.8.2.2 of IEEE standard 802.16-2004/Cor1-2005“Part 16: Air Interface for Fixed and Mobile Broadband Wireless AccessSystems; Amendment 2: Physical and Medium Access Control Layers forCombined Fixed and Mobile Operation in Licensed Bands and Corrigendum 1”(hereinafter, referred to as Reference Document 2).

Referring to FIGS. 4 to 7, pilot overhead is large when the allocationof the subcarrier is performed according to the PUSC or the FUSC. Inparticular, the overhead is larger when one transmission antenna is usedcompared to when at least two transmission antennas is used, inconsideration of pilot overhead per transmission antenna.

Table 1 shows the pilot overhead according to the number of transmissionantennas in each permutation method.

TABLE 1 Number of transmission antennas PUSC FUSC O-FUSC 1 14.28%(14.28%) 9.75% (9.75%) 11.1% (11.1%) 2 14.28% (7.14%) 9.75% (4.78%)11.1% (5.55%) 4 28.55% (7.14%) 18.09% (4.52%) 22.21% (5.55%)

The pilot overhead is a value obtained by dividing the number ofsubcarriers allocated to the pilots by the number of all subcarrierswhich are used. The value in parenthesis indicates the pilot overheadper transmission antenna. Further, according to Reference Document 2, iffour or three transmission antennas are used, mapping of the data to thesubchannels is performed after puncturing or truncation with respect tochannel encoded data.

Now, a pilot structure for multiple antennas according to the presentinvention will be described. Design criteria for an optimal pilotstructure are as follows.

(1) The pilot overhead in a single antenna is about 4 to 9% in atime-frequency domain. (2) One slot may include 48 subcarriers on twocontiguous OFDM symbols. (3) The pilot subcarriers are uniformlydistributed in the time-frequency domain. (4) The pilot overhead pertransmission antenna is similarly maintained and the total pilotoverhead is increased as the number of transmission antennas isincreased. In consideration of the total pilot overhead, the total pilotoverhead is equally maintained so as not to exceed 20% in at least threetransmission antennas. (5) Even when the number of transmission antennasis increased, mapping of the data to the subchannels is not influenced.

In order to derive a possible candidate group using the above-describedcondition, the following requirements can be expressed by equations andthe candidate group satisfying the requirements can be derived.(Nused−2*Np)mod Nsub=0  (1)(Nused−2*Np)mod Nsub=0  (2)(Nused−Np)mod Nsub=0  (3)0.04<=Np<=0.09  (4)Ng=Nfft−Nused−1(DC subcarrier)  (5)(Nused−Np)/Nsub.sym>=Nsch.pusc  (6)where, Nused denotes the number of used subcarriers, Np denotes thenumber of pilot subcarriers, Ng denotes the number of guard subcarriers,Nfft denotes a FFT size, Nsub denotes the number of subcarriers persubchannel, Nsub.sym denotes the number of subcarriers allocated to thesubchannel on one OFDM symbol, and Nsch.pusc denotes the number ofsubchannels which can be generated in the existing DL-PUSC.

Table 2 shows 26 candidate groups according to the design criteria.

TABLE 2 # of # of # of Pilot FFT Used Pilot Guard Subch - 1 Subch - 2Subch - 4 Overhead - 1 Data Guard Pilot # Size SCs SCs SCs antennaantennas antennas antenna Ratio Ratio Spacing  (1) 2048 1658 72 391 6663 60 4.3478 80.859 19.092 11.5  (2) 2048 1656 96 391 65 61 57 5.797180.859 19.092 8.625  (3) 2048 1680 72 367 67 64 61 4.2857 82.031 17.9211.6667  (4) 2048 1680 96 367 66 62 58 5.7143 82.031 17.92 8.75  (5)2048 1680 120 367 65 60 55 7.1429 82.031 17.92 7  (6) 2048 1704 72 34368 65 62 4.2254 83.203 16.748 11.833  (7) 2048 1704 96 343 67 63 595.6338 83.203 16.748 8.875  (8) 2048 1704 120 343 66 61 56 7.0423 83.20316.748 7.1  (9) 2048 1728 72 319 69 66 63 4.1667 84.375 15.576 12 (10)2048 1728 96 319 68 64 60 5.5556 84.375 15.576 9 (11) 2048 1728 120 31967 62 57 6.9444 84.375 15.576 7.2 (12) 2048 1728 144 319 66 60 54 8.333384.375 15.576 6 (13) 2048 1752 72 295 70 67 64 4.1096 85.547 14.40412.1667 (14) 2048 1752 96 295 69 65 61 5.4795 85.547 14.404 9.125 (15)2048 1752 120 295 68 63 58 6.8493 85.547 14.404 7.3 (16) 2048 1752 144295 67 61 55 8.2192 85.547 14.404 6.08 (17) 2048 1776 72 271 71 68 654.0541 86.719 13.232 12.333 (18) 2048 1776 96 271 70 66 62 5.4054 86.71913.232 9.25 (18) 2048 1776 120 271 69 64 59 6.7568 86.719 13.232 7.4(20) 1024 816 48 207 32 30 28 5.8824 79.688 20.215 8.5 (21) 1024 840 48183 33 31 29 5.7143 82.031 17.871 8.75 (22) 1024 864 48 159 34 32 305.5556 84.375 15.527 8 (23) 1024 864 72 159 33 30 27 8.3333 84.37515.527 6 (24) 1024 888 48 135 35 33 31 5.4054 86.719 13.184 9.25 (25)1024 888 72 135 34 31 28 8.1081 86.719 13.184 6.1665 (26) 512 432 24 7917 16 15 5.5556 84.375 15.43 9

The used subcarriers (used SCs), the pilot subcarriers (Pilot SCs) andthe guard subcarriers (Guard SCs) are obtained according to the FFTsize, and the subchannel according to the number of transmissionantennas is obtained. The used subcarrier is a value excluding a DCsubcarrier. For example, in the case (1), the number of subchannels inone transmission antenna is (the number of used subcarriers−the numberof pilot subcarriers)/(the number of subcarriers allocated to thesubchannels on one OFDM symbol)=1656−72/24=66.

FIG. 8 is a graph showing cases of Table 2 by a ratio of pilot overheadand a ratio of guard subcarriers. Referring to FIG. 8, among cases (1)to (26) shown in Table 2, in consideration of the system bandwidth orthe FFT size, it can be seen that cases which are most suitable for 5 to20 MHz or 512 to 2048 FFT size in the same design criteria are cases(10), (22) and (26). In Table 2, the cases (10), (22) and (26) have aspacing of 9 pilots. That is, in one embodiment of the presentinvention, the arrangement of the pilots at a spacing of 9 subcarriersis provided.

Table 3 shows the subcarrier allocation.

TABLE 3 Value Parameter 2048 1024 512 Comments # of DC Subcarriers 1 1 1Index 1024/512/256 (counting from 0) # of Guard Subcarriers, 160 80 40Left # of Guard Subcarriers, 159 79 39 Right # of Used Subcarriers 1729865 433 Number of all subcarriers used within a (N_(used)) symbol,including pilots & a DC carrier # of Pilot Subcarriers 96 48 24(N_(pilot)) Pilot Subcarrier Index 18k + 9m₁ + 3(└n_(s)/2┘mod 3) n_(s)denotes a symbol index, in which k = 0.1, . . ., N_(pilot) index “0”should be the first symbol of m_(s) = (n_(s) + i)mod 2, i = 0.1 thecurrent zone. DC carrier is excluded for the calculation of the pilotsubcarrier index # of Data Subcarriers 1632 816 408 (in each symbol persubch.) (24) (24) (24)

The subchannels are mapped to subcarriers excluding the subcarriers usedafter allocating the pilot subcarriers. At this time, a general PUSC orFUSC permutation method is applicable.

According to the above mentioned method, it is possible to improvethroughput by 6 to 13% in the PUSC/FUSC. For example, while 60subchannels can be obtained in the PUSC and 64 subchannels can beobtained in the FUSC according to the prior art, 68 subchannels can beobtained according to the above mentioned method. If a new permutationmethod is applied in consideration of the multiple antennas, capabilitycan be prevented from deteriorating due to data puncturing ortruncation.

The equation shown in the pilot subcarrier index of Table 3 represents apilot index Pi of an i^(th) antenna. This is expressed by the followingequation again.

Equation 1P ₁=18k+9m _(i)+3(|n _(s)/2 |mod 3)where, k=0, 1, . . . , Npilot, mi=(ns+i) mod 2, i=0, 1, |n| denotes aninteger smaller than n. In Equation 1, a factor “18” indicates that thenumber of subcarriers of the subchannel on one OFDM symbol is 18, afactor “9” indicates that the pilot subcarriers are arranged at aspacing of 9 subcarriers, and a factor “3” indicates that thesubcarriers are shifted at a spacing of three subcarriers in the unit ofa slot. In this case, each slot occupies 2 consecutive OFDM symbols intime domain.

FIG. 9 is an example of the pilot arrangement of two transmissionantennas. Referring to FIG. 9, one slot includes 72 subcarriers on twocontiguous OFDM symbols, and the pilot subcarriers for a first antenna(antenna 0) and the pilot subcarriers for a second antenna (antenna 1)are arranged at a spacing of 9 subcarriers on each OFDM symbol. Inaddition, on the first OFDM symbol and the second OFDM symbol, the pilotsubcarriers for the first antenna (antenna 0) and the pilot subcarriersfor the second antenna (antenna 1) are alternately arranged.

In a second slot, the pilot subcarriers are shifted from the pilotsubcarriers allocated to the first slot by three subcarriers. In a thirdslot, the pilot subcarriers are shifted from the pilot subcarriersallocated to the second slot by three subcarriers. As a result, the samepilot arrangement is repeated in every three slots.

In the pilot arrangement, the pilots can be moved at a constant spacingin the time domain or the frequency domain, and thus, do not haveabsolute positions. The pilot subcarriers may be shifted by at aconstant time interval or/and a subcarrier spacing while a spacingbetween the pilot subcarriers is maintained.

FIG. 10 is an example of the pilot arrangement of four transmissionantennas. Referring to FIG. 10, the pilot subcarriers for fourtransmission antennas (antenna 0, antenna 1, antenna 2, and antenna 3)are contiguous in the frequency domain or in the time domain. The pilotsubcarriers for the transmission antennas are arranged at a spacing of12 subcarriers.

In a second slot, the pilot subcarriers are shifted from the pilotsubcarriers allocated to the first slot by six subcarriers. As a result,the same pilot arrangement is repeated every two slots.

In the four transmission antennas, since the number of pilots is largerthan the number of the two transmission antennas, the pilot spacing iswider than the pilot spacing in the two transmission antennas and thecyclic period of the repeated slot is decreased.

FIG. 11 illustrates the pilot arrangement of three or four transmissionantennas by equation. Referring to FIG. 11, G₀, G₁, G₂, and G₃ aredefined in a 2×2 region including two OFDMs (i=0, 1) and twosubcarriers. For example, in the four transmission antennas, the pilotcarriers are arranged as shown in Table 4, which is equal to thearrangement of FIG. 10.

TABLE 4 Antenna 0 Antenna 1 Antenna 2 Antenna 3 Pilot Subcarrier G₀ G₁G₂ G₃ Index

In the three transmission antennas, the pilot subcarriers are arrangedas shown in Table 5.

TABLE 5 └n₂/2┘ mod 2 0 1 2 Pilot Antenna 0 G₀, G₃ G₂ G₁ SubcarrierAntenna 1 G₁ G₀, G₃ G₂ Index Antenna 2 G₂ G₁ G₀, G₃

In order to express the pilot arrangement for the four transmissionantennas by equation, six pilot subcarriers P^(i) ₀, P^(i) ₁, P^(i) ₂,P^(i) ₃, P^(i) ₄, and P^(i) ₅ are considered on OFDM symbols (i=0, 1),as shown in Table 6.

TABLE 6 m_(i) = 0 m_(i) = 1 k = even P^(i) ₀ = {PilotSub(k, m_(i))}P^(i) ₂ = {PilotSub(k, m_(i)) + 3} P^(i) ₁ = {PilotSub(k, m_(i)) + 1} k= odd P^(i) ₃ = {PilotSub(k, m_(i)) − 5} P^(i) ₄ = {PilotSub(k, m_(i)) −3} P^(i) ₅ = {PilotSub(k, m_(i)) − 2} where, PilotSub(k, m_(i)) = 18k +9m_(i) + 6( |n₂/2| mod2)

Accordingly, in one slot, a pilot subcarrier set may be mapped to thefour pilot subcarriers G₀, G₁, G₂, and G₃ as shown in Equation 2.

Equation 2G _(i) =P ^(i) ₀ +P ^(i) ₂ +P ^(i) ₄ and G _(i+2) =P ^(i) ₁ +P ^(i) ₃ +P^(i) ₅

-   -   where, i=0, 1.

According to the pilot structure, the pilot overhead satisfying thedesign criteria is obtained and the overhead is decreased from the priorart by 5% or more. The pilot overhead is 5.55% in the two transmissionantennas, 5.55% in the three transmission antennas, and 4.16% in thefour transmission antennas.

Even when the number of transmission antennas is increased, the mappingof data to the subchannels is not influenced. Accordingly, thepermutation method can be simply performed.

In a method of generating distributed subchannels used in the existingIEEE 802.16-2004 standard, pilot subcarriers for the first antenna andthe second antenna are first allocated and subchannels are constructedusing residual subcarriers. In a third antenna and a fourth antenna,since the pilot subcarriers are allocated and used using the allocatedsubchannels, the subchannels of the same number are constructedregardless of the number of antennas. However, according to the pilotstructure, a necessary number of pilot subcarriers are allocatedaccording to the number of antennas and the subchannels are constructedusing the residual subcarriers. Accordingly, the number of subchannelsis increased while optimizing the pilot overhead.

FIG. 12 is another example of the pilot arrangement of two transmissionantennas. Referring to FIG. 12, the pilot subcarriers for one antennaare arranged on one ODFM symbol at a spacing of nine subcarriers. Thatis, the pilot subcarriers for the first antenna (antenna 0) are arrangedon the first OFDM symbol at a spacing of nine subcarriers, and the pilotsubcarriers for the second antenna (antenna 1) are arranged on thesecond OFDM symbol at a spacing of nine subcarriers.

In a second slot, the pilot subcarriers are arranged by shifting thepilot subcarriers allocated to the first slot by three subcarriers. In athird slot, the pilot subcarriers are arranged by shifting the pilotsubcarriers allocated to the second slot by three subcarriers. As aresult, the same pilot arrangement is repeated every three slots.

FIG. 13 is another example of the pilot arrangement of two transmissionantennas. Referring to FIG. 13, the arrangement of the pilot subcarriersin one slot is equal to that of the embodiment shown in FIG. 9. However,in a second slot, the pilot subcarriers are shifted from the pilotsubcarriers by four subcarriers. As a result, the same pilot arrangementis repeated every two slots.

Referring to FIG. 14, the pilot subcarriers for the transmissionantennas (antenna 0, antenna 1, antenna 2, and antenna 3) are arrangedat a spacing of 12 subcarriers and are arranged by shifting thefrequency position in the unit of two OFDM symbols in every slot. In asecond slot, the pilot subcarriers are shifted from the pilotsubcarriers allocated to the first slot by six subcarriers. As a result,the same pilot arrangement is repeated every two slots.

FIG. 15 is another example of the pilot arrangement of four transmissionantennas (antenna 0, antenna 1, antenna 2, and antenna 3). Referring toFIG. 15, the pilot subcarriers for the transmission antennas arearranged at a spacing of 12 subcarriers and the pilot subcarriers forthe antennas are alternately arranged between two subsequent OFDMsymbols in one slot. By alternately arranging 2 pilot subcarriers in twosubsequent OFDM symbols, that is, by equally allocating each of pilotsubcarriers for each antenna in 1 OFDM symbol, transmission power foreach antenna can be balanced in certain time instance. In FIG. 15, twopilot subcarriers for respective two antennas are paired, and each ofthe two pairs of pilot subcarriers are alternatively allocated in twoOFDM symbols.

Hereinafter, an efficient pilot allocation structure according to anembodiment of the present invention is described. A structure forallocating the pilot subcarriers efficiently shifts the pilot structurebetween contiguous cells in order to avoid collision between pilotsubcarriers.

In the following embodiment of the present invention, as an example, abasic resource block unit is composed of 18 subcarriers (verticalaxis)*six OFDM symbols (horizontal axis). However, the method ofallocating the pilot subcarriers according to the present invention isalso applicable to other basic resource block units by expanding thesame method to a subframe or a frame.

In the following embodiment, the horizontal axis denotes a set of OFDMsymbols in the time domain and the vertical axis denotes the subcarriersin the frequency domain. P1, P2, P3, P4, P5, P6, P7 and P8 denote thepilot subcarriers corresponding to the antennas 1, 2, 3, 4, 5, 6, 7 and8, respectively.

FIGS. 16˜18 are pilot subcarrier allocation structures in a system usingeight antennas according to an embodiment of the present invention. Thepilots of the first transmission antenna (P1) and the pilots of thesecond transmission antenna (P2), the pilots of the third transmissionantenna (P3) and the pilots of the fourth transmission antennas (P4),the pilots of the fifth transmission antenna (P5) and the pilots of thesixth transmission antennas (P6), and the pilots of the seventhtransmission antenna (P7) and the pilots of the eighth transmissionantennas (P8) are respectively paired with each other and allocated soas to be contiguous to each other on two OFDM symbols, as shown in FIGS.16˜18. Further, the structure for consecutively allocating the pilotsubcarriers for the antennas at a spacing of 18 subcarriers in thefrequency axis is provided. That is, the pilot subcarriers are allocatedat a spacing of a subchannel including 18 subcarriers.

Specifically, FIG. 16 shows a pilot pattern shifted and allocated at thesubcarrier spacing of 2 per unit of two OFDM symbols. FIG. 17 shows apilot pattern shifted and allocated at the subcarrier spacing of 6 perunit of two OFDM symbols. FIG. 18 shows a pilot pattern shifted andallocated at the subcarrier spacing of 6 per unit of two OFDM symbols,as in FIG. 17, but having additional 1 subcarrier offset.

In an aspect of the present invention, the pilot subcarriers for theantennas are equally allocated in each OFDM symbol, and the pilotsubcarrier pattern is shifted and allocated at a predetermined spacingin a unit of two OFDM symbols. In another aspect of the presentinvention, the pilots for eight transmission antennas may be allocatedto be shifted by a predetermined subcarrier spacing per each of thepilots for the antennas or pilot subcarrier pairs, instead of beingcontiguously allocated, as shown in FIGS. 16˜18.

The pilot allocation may have the same allocation structure regardlessof a regular subframe or an irregular subframe. A part of the pilotsallocated according to an embodiment of the present invention may beused in a common pilot and the other part thereof may be used in adedicated pilot. All the allocated pilots may be applied to thededicated pilot or the common pilot. This embodiment is characterized inthat the pilots for all antennas are equally allocated in one OFDMsymbol for power balancing of the antenna per OFDM symbol. In the methodof allocating the pilot subcarriers according to the above-describedembodiment, a shift offset for pilot allocation may vary according tocells.

FIGS. 19 and 20 show that a pilot subcarrier allocation offset variesaccording to cells in a 4-Tx system and an 8-Tx system. In particular,FIG. 19 shows that the pilot subcarrier allocation offset is differentlyset to cells A, B and C in the 4-Tx system. And FIG. 20 shows that thepilot subcarrier allocation offset is differently set to the cells A, Band C in the 8-Tx system.

That is, the pilot allocation structure can be differently applied tothe cells A, B and C. The applied shift offset value is applied in thenumber of subcarriers of 1 to 18. The 18 subcarriers correspond to thebasic resource block size.

The shift offset value may be an integral multiple of the basic resourceblock size. Although the shift offset value is applied to the frequencyaxis in the present embodiment, it may be applied to the time axis.

If the number of contiguous cells is three or more and theabove-described principle is expanded and applied, the structures shownin FIGS. 19 and 20 may be repeatedly used or may be applied in a stateof being shifted by a predetermined subcarrier offset or a predeterminedOFDM symbol offset.

FIG. 21 shows another embodiment of the 8-Tx system shown in FIG. 20.FIG. 22 shows a pilot subcarrier allocation pattern according to anotherembodiment of the present invention.

In the eight transmission antennas, the pilots may not be allocated forall eight antennas due to the pilot overhead. That is, when the pilotsare allocated only for the antennas 1, 2, 3 and 4, the overhead can bereduced. For example, such pilot allocation structure is applicable to aSFBC-CDD method. In the pilot allocation pattern shown in FIG. 22, amethod of shifting and using the pilot pattern between contiguous cellsis equal to that of the above-described embodiment.

FIGS. 23˜25 show pilot subcarrier allocation patterns in a 2-Tx systemaccording to another embodiment of the present invention. The principleaccording to the above-described embodiment is equally applied to thepilot patterns shown in FIGS. 23˜25. Thus, the pilot subcarriers for thefirst antenna and the pilot subcarriers for the second antenna formpairs and are contiguously arranged in two contiguous OFDM symbol areas.The pilot subcarriers for all antennas are set to be included in each ofthe OFDM symbol areas by the same number in order to equally set thetransmission power allocated to the antennas at a specific time.

Preferably, the pilot subcarrier pairs are allocated at a spacing ofnine subcarriers, in consideration of optimal granularity for the pilotsubcarrier allocation in a coherence bandwidth. Further, the pairs ofthe pilot subcarriers for the first antenna and the pilot subcarriersfor the second antenna are shifted by a predetermined number ofsubcarriers in a unit of two OFDM symbols. Although FIGS. 23 and 24 showthat the shifted subcarrier spacing is a spacing corresponding to threesubcarriers, for example, the subcarrier indexes are increased by threesubcarriers, the shifted subcarrier spacing may be a predeterminedmultiple of three subcarriers, for example, six subcarriers, and may beapplied such that the indexes are decreased by three subcarrier indexes.FIG. 25 shows that the shifted subcarrier spacing is set to be 6subcarriers. FIG. 25 may also be construed that the shifted subcarrierspacing is applied such that the indexes are decreased by 3 subcarrierindexes.

As described above, the reason why the frequency-domain shift is set ata spacing corresponding to three subcarriers or the multiple of threesubcarriers is because the pilot subcarriers are allocated in a unit ofnine subcarriers and the pilot subcarrier allocation pattern is repeatedwith a predetermined period. The pilot pattern shown in FIGS. 23˜25 maybe equally and repeatedly applied to the time/frequency domain in theframe or the subframe. The positions of the pilots for the antenna 1 andthe positions of the pilots for the antenna 2 may be exchanged withoutdeparting from the principal of this embodiment.

In the pilot allocation structure shown in FIG. 23, the pilot allocationindexes for the antennas are represented in detail as follows.

<Pilot Allocation Index for FIG. 23>

Antenna 1—

-   -   18 k+1 when s is 0    -   18 k+10 when s is 1    -   18 k+4 when s is 2    -   18 k+13 when s is 3    -   18 k+7 when s is 4    -   18 k+16 when s is 5

Antenna 2—

-   -   18 k+10 when s is 0    -   18 k+1 when s is 1    -   18 k+13 when s is 2    -   18 k+4 when s is 3    -   18 k+16 when s is 4    -   18 k+7 when s is 5    -   k: subcarrier index (k=0, 1, . . . ),    -   s: [OFDM symbol index] mod 6    -   (OFDM symbol index=0, 1, 2, . . . )

And, in the pilot allocation structure shown in FIG. 24, the pilotallocation indexes for the antennas may be represented in detail asfollows.

<Pilot Allocation Index for FIG. 24>

Antenna 1—

-   -   18 k when s is 0    -   18 k+9 when s is 1    -   18 k+3 when s is 2    -   18 k+12 when s is 3    -   18 k+6 when s is 4    -   18 k+15 when s is 5

Antenna 2—

-   -   18 k+9 when s is 0    -   18 k when s is 1    -   18 k+12 when s is 2    -   18 k+3 when s is 3    -   18 k+15 when s is 4    -   18 k+6 when s is 5    -   k: subcarrier index (k=0, 1, . . . ),    -   s: [OFDM symbol index] mod 6    -   (OFDM symbol index=0, 1, 2, . . . )

In addition, in the pilot allocation structure shown in FIG. 25, thepilot allocation indexes for the antennas may be represented in detailas follows.

<Pilot Allocation Index for FIG. 25>

Antenna 1—

-   -   18 k when s is 0    -   18 k+9 when s is 1    -   18 k+6 when s is 2    -   18 k+15 when s is 3    -   18 k+3 when s is 4    -   18 k+12 when s is 5

Antenna 2—

-   -   18 k+9 when s is 0    -   18 k when s is 1    -   18 k+15 when s is 2    -   18 k+6 when s is 3    -   18 k+12 when s is 4    -   18 k+3 when s is 5    -   k: subcarrier index (k=0, 1, . . . ),    -   s: [OFDM symbol index] mod 6    -   (OFDM symbol index=0, 1, 2, . . . )

In the pilot allocation structure according to the above-describedembodiment, if a preamble OFDM symbol is transmitted to a foremostportion of the subframe in a predetermined period, the pilot subcarriersmay be changed so as to be applied to a second OFDM symbol and thesubsequent OFDM symbols.

FIGS. 26 and 27 show pilot subcarrier allocation patterns in a 4-Txsystem according to another embodiment of the present invention. Thebasic pilot allocation methods of FIGS. 26 and 27 are equal to those ofthe above-described embodiments. However, in the present embodiment, thepilot subcarriers for the four antennas are allocated to be contiguousto each other in contiguous four OFDM symbol areas.

In the pilot allocation structure shown in FIG. 26, the pilot allocationindexes for the antennas may be represented in detail as follows.

<Pilot Allocation Index for FIG. 26>

Antenna 1—

-   -   18 k+1 when s is 0    -   18 k+10 when s is 1    -   18 k+4 when s is 2    -   18 k+13 when s is 3    -   18 k+7 when s is 4    -   18 k+16 when s is 5

Antenna 2—

-   -   18 k+10 when s is 0    -   18 k+1 when s is 1    -   18 k+13 when s is 2    -   18 k+4 when s is 3    -   18 k+16 when s is 4    -   18 k+7 when s is 5    -   k: subcarrier index (k=0, 1, . . . ),    -   s: [OFDM symbol index] mod 6    -   (OFDM symbol index=0, 1, 2, . . . )

Antenna 3—

-   -   18 k+4 when s is 0    -   18 k+13 when s is 1    -   18 k+7 when s is 2    -   18 k+16 when s is 3    -   18 k+10 when s is 4    -   18 k+1 when s is 5

Antenna 4—

-   -   18 k+13 when s is 0    -   18 k+4 when s is 1    -   18 k+16 when s is 2    -   18 k+7 when s is 3    -   18 k+1 when s is 4    -   18 k+10 when s is 5    -   k: subcarrier index (k=0, 1, . . . ),    -   s: [OFDM symbol index] mod 6    -   (OFDM symbol index=0, 1, 2, . . . )

In the pilot allocation structure shown in FIG. 27, the pilot allocationindexes for the antennas may be represented in detail as follows.

<Pilot Allocation Index for FIG. 27>

Antenna 1—

-   -   18 k when s is 0    -   18 k+9 when s is 1    -   18 k+3 when s is 2    -   18 k+12 when s is 3    -   18 k+6 when s is 4    -   18 k+15 when s is 5

Antenna 2—

-   -   18 k+9 when s is 0    -   18 k when s is 1    -   18 k+12 when s is 2    -   18 k+3 when s is 3    -   18 k+15 when s is 4    -   18 k+6 when s is 5    -   k: subcarrier index (k=0, 1, . . . ),    -   s: [OFDM symbol index] mod 6    -   (OFDM symbol index=0, 1, 2, . . . )

Antenna 3—

-   -   18 k+3 when s is 0    -   18 k+12 when s is 1    -   18 k+6 when s is 2    -   18 k+15 when s is 3    -   18 k+9 when s is 4    -   18 k when s is 5

Antenna 4—

-   -   18 k+12 when s is 0    -   18 k+3 when s is 1    -   18 k+15 when s is 2    -   18 k+6 when s is 3    -   18 k when s is 4    -   18 k+9 when s is 5    -   k: subcarrier index (k=0, 1, . . . ),    -   s: [OFDM symbol index] mod 6    -   (OFDM symbol index=0, 1, 2, . . . )

FIG. 28 shows a pilot subcarrier allocation pattern in a 4-Tx systemaccording to another embodiment of the present invention. The principledescribed in the above-described embodiments is similarly applied to thepilot pattern of FIG. 28. However, FIG. 26 shows an example that thepairs of pilots are arranged at the spacing of two subcarriers when thesubcarriers for the first antenna and the subcarriers for the secondantenna form a pair of pilots, and the subcarriers for the third antennaand the subcarriers for the fourth antenna form another pair of pilots.That is, in the present embodiment, the allocation may be performed suchthat two pairs of pilots are allocated to be contiguous to each other ornot to be contiguous to each other.

The above-described functions may be performed by a processor such as amicroprocessor, a controller, a microcontroller or an applicationspecific integrated circuit (ASIC) which is coded so as to perform thefunctions. The design, development and implementation of the code areapparent to those skilled in the art on the basis of the description ofthe present invention.

A method of allocating pilot subcarriers according to the presentinvention is applicable to an IEEE 802.16m system. As described above, abasic principle such as pilot arrangement for equally allocatingtransmission power to antennas or pilot shift pattern setting is alsoapplicable to other wireless communication systems by the same method.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method of transmitting pilot signals byallocating pilot subcarriers for use in downlink or uplink communicationin a multiple-input multiple-output (MIMO) antenna system usingorthogonal frequency division multiplexing (OFDM) modulation, the methodcomprising: allocating first pilot signals for a first antenna in timeand frequency domains comprising OFDM symbols in the time domain andsubcarriers in the frequency domain; allocating second pilot signals fora second antenna in the time and frequency domains; and transmitting thefirst and the second pilot signals to a receiver, wherein each firstpair consisting of one among the first pilot signals and one among thesecond pilot signals is allocated in two contiguous OFDM symbols,wherein the first pairs allocated within the same OFDM symbols areallocated in a unit of 9 subcarriers in the frequency domain, whereinthe first pilot signal and the second pilot signal are alternatelyallocated within each OFDM symbol, and wherein each of the first pilotsignals separated by two OFDM symbols is shifted by multiple of threesubcarriers, and each of the second pilot signals separated by two OFDMsymbols is shifted by multiple of three subcarriers.
 2. The method ofclaim 1, wherein each OFDM symbol comprises first and second pilotsignals allocated thereon.
 3. The method of claim 2, wherein numbers ofeach of the first and second pilot signals allocated in each OFDM symbolare the same.
 4. The method of claim 1, further comprising: allocatingthird pilot signals for a third antenna in the time and frequencydomains; allocating fourth pilot signals for a fourth antenna in thetime and frequency domains; and transmitting the second and the thirdpilot signals to the receiver, wherein each second pair consisting ofone among the third pilot signals and one among the fourth pilot signalsis allocated in two contiguous OFDM symbols, wherein the second pairsallocated within the same OFDM symbols are allocated in a unit of 9subcarriers in the frequency domain, and wherein the third pilot signaland the fourth pilot signal are alternately allocated within each OFDMsymbol.
 5. The method of claim 4, wherein the allocated first pilotsignals are adjacent to the allocated third pilot signals in thefrequency domain and the allocated second pilot signals are adjacent tothe allocated fourth pilot signals in the frequency domain.
 6. Themethod of claim 4, wherein each of the third pilot signals separated bytwo OFDM symbols is shifted by multiple of 3 subcarriers, and each ofthe fourth pilot signals separated by two OFDM symbols is shifted bymultiple of 3 subcarriers to differentiate frequency selectivity.
 7. Themethod of claim 1, wherein the time and frequency domains are used inthe uplink or downlink communication.
 8. The method of claim 1, whereinthe time and frequency domains are used in a type of a full usage ofsubchannels (FUSC) permutation mode.
 9. The method of claim 1, whereinthe time and frequency domains are used in a type of an adaptivemodulation and coding (AMC) permutation mode.
 10. The method of claim 1,wherein a starting location of the first pilot signals allocated in afirst OFDM symbol is offset by one subcarrier.
 11. The method of claim5, wherein each OFDM symbol comprises first, second, third and fourthpilot signals allocated thereon.
 12. The method of claim 11, whereinnumbers of each of the first, second, third and fourth pilot signalsallocated in each OFDM symbol are the same.
 13. A wireless communicationsystem using orthogonal frequency division multiplexing (OFDM)modulation for downlink or uplink communication, the wirelesscommunication system comprising: a multiple-input multiple-output (MIMO)antenna; an OFDM modulator operably connected to the MIMO antenna; and aprocessor operably connected to the OFDM modulator, wherein theprocessor is configured to allocate first pilot signals for a firstantenna and second pilot signals for a second antenna in time andfrequency domains comprising OFDM symbols in the time domain andsubcarriers in the frequency domain, wherein each first pair consistingof one among the first pilot signals and one among the second pilotsignals is allocated in two contiguous OFDM symbols, wherein the firstpairs allocated within the same OFDM symbols are allocated in a unit of9 subcarriers in the frequency domain, wherein the first pilot signaland the second pilot signal are alternately allocated within each OFDMsymbol, and wherein each of the first pilot signals separated by twoOFDM symbols is shifted by multiple of three subcarriers, and each ofthe second pilot signals separated by two OFDM symbols is shifted bymultiple of three subcarriers.
 14. The wireless communication system ofclaim 13, wherein each OFDM symbol comprises first and second pilotsignals allocated thereon.
 15. The wireless communication system ofclaim 14, wherein numbers of each of the first and second pilot signalsallocated in each OFDM symbol is the same.
 16. The wirelesscommunication system of claim 13, wherein the processor is furtherconfigured to allocate third pilot signals for a third antenna andfourth pilot signals for a fourth antenna in the time and frequencydomains, wherein each second pair consisting of one among the thirdpilot signals and one among the fourth pilot signals is allocated in twocontiguous OFDM symbols, wherein the second pairs allocated within thesame OFDM symbols are allocated in a unit of 9 subcarriers in thefrequency domain, and wherein the third pilot signal and the fourthpilot signal are alternately allocated within each OFDM symbol.
 17. Thewireless communication system of claim 16, wherein the allocated firstpilot signals are adjacent to the allocated third pilot signals in thefrequency domain and the allocated second pilot signals are adjacent tothe allocated fourth pilot signals in the frequency domain.
 18. Thewireless communication system of claim 16, wherein each of the thirdpilot signals separated by two OFDM symbols is shifted by multiple of 3subcarriers, and each of the fourth pilot signals separated by two OFDMsymbols is shifted by multiple of 3 subcarriers to differentiatefrequency selectivity.
 19. The wireless communication system of claim13, wherein the time and frequency domains are used in the uplink ordownlink communication.
 20. The wireless communication system of claim13, wherein the time and frequency domains are used in a type of a fullusage of subchannels (FUSC) permutation mode.
 21. The wirelesscommunication system of claim 13, wherein the time and frequency domainsare used in a type of an adaptive modulation and coding (AMC)permutation mode.
 22. The wireless communication system of claim 13,wherein a starting location of the first pilot signals allocated in afirst OFDM symbol is offset by one subcarrier.
 23. The wirelesscommunication system of claim 17, wherein each OFDM symbol comprisesfirst, second, third and fourth pilot signals allocated thereon.
 24. Thewireless communication system of claim 23, wherein numbers of each ofthe first, second, third and fourth pilot signals allocated in each OFDMsymbol are the same.
 25. The wireless communication system of claim 13,wherein the processor comprises: a channel encoder encoding an inputstream and building a coded word; a mapper mapping the coded word to asymbol representing a position on signal constellation; and amultiple-input multiple-output (MIMO) processor processing the symbol.26. The method of claim 1, wherein an order of the first pilot signaland the second pilot signal allocated within each of the first pairschanges per each of the first pairs allocated within the same OFDMsymbols.
 27. The method of claim 4, wherein an order of the third pilotsignal and the fourth pilot signal within each of the second pairschanges per each of the second pairs allocated within the same OFDMsymbols.
 28. The wireless communication system of claim 13, wherein anorder of the first pilot signal and the second pilot signal allocatedwithin each of the first pairs changes per each of the first pairsallocated within the same OFDM symbols.
 29. The method of claim 16,wherein an order of the third pilot signal and the fourth pilot signalallocated within each of the second pairs changes per each of the secondpairs allocated within the same OFDM symbols.